04 Mechanical Properties of Materials Free Online FlashCards

What is engineering stress under a simple axial load?

Stress is internal force per unit area: $\sigma = \frac{F}{A_0}$, where $F$ is applied force and $A_0$ is the original cross-sectional area.

How is engineering tensile strain defined?

Engineering tensile strain is relative length change: $\varepsilon = \frac{L-L_0}{L_0} = \frac{\Delta L}{L_0}$. It has no units.

What does Young’s modulus measure?

Young’s modulus $E$ measures stiffness. In the initial approximately linear region, $\sigma = E\varepsilon$; a stiffer material deforms less under a given elastic stress.

What does yield strength indicate?

Yield strength marks the onset of significant permanent deformation. If a clear yield point is absent, it is commonly determined by the 0.2% offset method.

How do elastic and plastic deformation differ?

Elastic deformation is recoverable when the load is removed; plastic deformation leaves a permanent change in shape.

How can grain refinement strengthen a material?

Grain boundaries obstruct dislocation motion, so smaller grains often increase a material’s yield strength; the Hall–Petch relation describes this trend over many grain-size ranges.

What condition signals unstable crack growth in linear-elastic fracture mechanics?

Fracture toughness measures resistance to crack growth under specified conditions. In linear-elastic fracture mechanics, unstable growth occurs when $K_I = Y\sigma\sqrt{\pi a} \geq K_{IC}$.

What is fatigue, and why can it cause failure below yield strength?

Fatigue is damage from repeated or fluctuating loading. A component may fail after many cycles even when stress is below the yield strength measured in a single loading event.

What relationship does an S–N curve show?

An S–N curve plots cyclic stress amplitude against cycles to failure.

What is creep?

Creep is time-dependent deformation under sustained stress. It is especially important at elevated temperatures, though some materials can creep at ordinary service temperatures.

What are the three stages of a typical creep curve?

Primary creep has a decreasing rate, secondary creep proceeds at a nearly steady rate, and tertiary creep accelerates, often as damage accumulates before rupture.

What does hardness measure, and what is one limit of the measurement?

Hardness is resistance to localized indentation or scratching. It is useful for comparison and quality control, but does not universally substitute for tensile strength, toughness, or fatigue testing.